1. Technical Field
The present invention relates to ion sources for molecular ions and to gas-filled ion mobility analyzers, and a related apparatus and method therefore.
2. Related Art
Related art ion sources must produce ions gently in order to not fragment the molecules of interest. This may be done via electrospray ion sources, in which a conductive liquid is pushed through a small capillary. At the capillary exit, small droplets are formed, of which a large number is charged and accelerated to form a diverging beam. If some of these charged droplets contain a molecule of interest and the liquid of the droplet evaporates, the droplet charge can remain on the molecules. Such ion sources include:
1. Related art “electrospray ion sources” in which molecules of interest are dissolved in the conductive liquid with which the ion source is fed. This requires that (a) the molecules of interest are capable of being dissolved in the liquid, (e.g., a mixture of different solvents), and (b) the electrospray ion source is capable of efficiently producing charged droplets from the liquid.
2. “Droplet pickup ion sources” that allow sample treatment to be used for many applications. Such “droplet pickup ion sources” include an electrospray ion source that forms charged droplets from a mixture of solvents in which no appreciable quantity of the molecules of interest is dissolved while the molecules of interest are incorporated into the droplets when they pass through a “pickup region” in which the molecules of interest exist. In this “pickup region”, the charged droplets can incorporate these molecules if (a) the molecules of interest exist in this “pickup region” as free molecules that can adsorb at the surfaces of the charged droplets that had been brought into this region (such free molecules can for instance be desorbed by a laser beam from a solid or liquid sample), if (b) the molecules of interest exist on the surface of a solid or liquid sample within the “pickup region” from where they can adsorb at the surfaces of the charged droplets directly when these interact with this surface and if (c) the molecules of interest have been dissolved in independently formed neutral droplets that can fuse with the charged droplets in the “pickup region”.
A related art version of the “droplet pickup ion sources” has been used, but only as ion sources for mass spectrometers.
In the related art mobility analyzers, electric fields drag clouds of molecular ions through a gas with an average drag velocity that increases with the magnitude of the electric field. The proportionality constant is the ion mobility that increases substantially with the inverse of the molecule cross section. Such mobility analyzers include:
1. “Differential Mobility Analyzers (DMA)” in which a substantially constant electric field drags ions substantially perpendicular to a gas stream. Depending on the time the ions of interest need to cross the gas stream, the gas flow moves the ions a shorter or a longer distance in a direction perpendicular to the electric field.
2. “Ion Mobility Spectrometers (IMS)” in which a substantially constant electric field drags ions through a stationary gas or through a gas that moves substantially parallel to the electric field. Depending on the average drag velocity of the ions of interest, these ions pass in different times through the region in which the electric field exists. This electric field is static, but can be pulsed in parts, or take the form of a traveling wave.
3. “Differential Mobility Spectrometers (DMS)”, wherein in a given gas, the mobilities of different ions vary with the magnitude of the electric field in different ways. In such a DMS, an electric RF-field drags ions back and forth substantially perpendicular to a gas stream between two electrodes. The waveform of the RF-field is chosen to be asymmetric. As a result, for a short time a high field drags the ions in a first direction and for a longer time a low field drags the ions in a second and opposite direction. Thus, the ions that can pass between the two electrodes are ions having a specific ratio between their high-field mobility and their low-field mobility, depending on the asymmetry of the RF-waveform. Such systems have been developed with electric fields formed between planar electrodes or between electrodes that form concentric cylinders.